SEATTLE, WASHINGTON—Even among cosmic heavyweights, the black hole in the core of galaxy M87 stands out. New observations reveal that the object weighs in at a whopping 6.6 billion suns, making it the most massive black hole for which a precise mass has ever been measured. "It could swallow our solar system whole," says astronomer Karl Gebhardt of the University of Texas, Austin, who presented the new results here Wednesday afternoon at the 217th meeting of the American Astronomical Society.

Previous estimates put the mass of the black hole at some 3 billion times the mass of our sun, still nearly 1000 times as massive as the black hole at the center of the Milky Way. To compensate for the blurring effects of Earth's atmosphere and obtain a more accurate measurement, Gebhardt and colleagues utilized the adaptive optics capability of the 8.1-meter Frederick C. Gillett Gemini Telescope on Mauna Kea, Hawaii. That enabled the astronomers to measure how fast stars orbit the black hole, which lies some 50 million light-years away, in the direction of the constellation Virgo. From the observed speeds—up to almost 500 kilometers per second—they could then calculate the hole's mass. "It's the most accurate mass estimate ever obtained" for a supermassive black hole, says Gebhardt.

Gebhardt says the black hole's event horizon—the edge from within nothing can escape, not even light—is four times as large as the orbit of Neptune, the outermost planet in our solar system.

Supermassive black holes like the one in galaxy M87 probably grow not only by feeding on infalling gas and stars but also by mergers of smaller black holes. Astronomer George Djorgovski of the California Institute of Technology in Pasadena used adaptive optics on the 10-meter Keck telescope, also on Mauna Kea, to reveal orbiting pairs of black holes in 16 distant galaxies. "I expect them to merge within a few million years from now," he says.

Djorgovski, who also presented his results at the Seattle meeting, suspects that M87—by far the largest and most massive galaxy in the nearby universe—may be the result of successive mergers of about a hundred smaller galactic building blocks. Stellar motions in the core of the giant galaxy do indeed suggest that it may have experienced a black hole merger in the not-too-distant past, says Gebhardt.

Finding the most massive black hole isn't just a highlight for the record books. Gebhardt says studying extreme black holes like the one in M87 gives astronomers their best chance of learning more about black hole physics in general. "In fact," he says, "future observations of this object might finally help us prove that what we call black holes are really black holes. Until now, there has been no direct observational evidence at all for the existence of event horizons," which are the true signatures of black holes.

That may change within 10 years or so, when astronomers hope to succeed in hooking up telescopes from all over the world that observe the universe at wavelengths shorter than 1 millimeter. According to Gebhardt, using such a "whole Earth submillimeter interferometer" may actually detect the silhouette of the black hole's event horizon against the galaxy's background glow.